Dominant 7th b5
7b5
0 - 4 - 6 - 10Dom7 with diminished 5th
About Dominant 7th b5
A dominant seventh flat five chord is a dominant with its fifth lowered — in C that is C, E, G♭ and B♭, written C7♭5. It has a property no other chord in the catalogue has: the same four notes spell the same chord a tritone away, which makes it the theoretical basis of the substitution jazz uses more than any other.
Notes and formula
The formula is 1 – 3 – ♭5 – ♭7, or 0 – 4 – 6 – 10 in semitones. The perfect fifth is replaced rather than joined.
The chord therefore contains two tritones: E to B♭, and C to G♭. Both are unstable, and the second is what makes the chord symmetrical.
The chord that is its own substitute
Spell C7♭5 — C, E, G♭, B♭ — then spell F♯7♭5: F♯, A♯, C, E. Written differently, they are the same four pitches. A dominant flat five and the dominant flat five a tritone away are one chord.
That is why the tritone substitution works so cleanly. Any C7 can be replaced by F♯7, and if both are played as flat-five chords the substitution changes nothing at all except the bass note.
How to play it
On guitar the movable form is x–3–4–3–x–x, sounding C–G♭–B♭. Adding the third on the second string gives the full chord and a stretch most players avoid.
On piano the useful voicing drops nothing: C–E–G♭–B♭ is only four notes and every one of them carries information, which is unusual for a dominant.
How it sounds
Ambiguous and weightless. With no perfect fifth, the chord has no anchor, and the symmetry means the ear cannot decide which note is the root without help from the bass.
That is the effect composers want from it: harmony that is clearly going somewhere without being clear where from.
Flat five, sharp eleven, altered
C7♯11 keeps the perfect fifth and adds the raised fourth above it — five notes, a Lydian dominant. C7♭5 has four and no fifth at all.
C7alt has both the fifth and the ninth altered. The flat five is the smallest step down that road: one alteration, one lost note, and the symmetry arrives for free.
Scales that contain it
The whole-tone scale contains every note and nothing that contradicts them, and its own symmetry matches the chord's. It is the default choice.
The altered scale and Lydian dominant both fit as well, and each adds a different ninth. Whole tone adds none, which is why it sounds the most weightless of the three.
Frequently asked questions
What notes are in a C7♭5 chord?
C, E, G♭ and B♭. On guitar, x–3–4–3–x–x for the shell.
Why is C7♭5 the same as F♯7♭5?
Because the chord is symmetrical. Spell both and you get the same four pitches with different names, which is exactly what makes tritone substitution work.
Is 7♭5 the same as 7♯11?
No. Sharp eleven keeps the perfect fifth and has five notes; flat five replaces it and has four. The altered note is the same pitch, but the chords sound different because of what is missing.